1. Field of the Invention
The present invention relates to a semiconductor device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As such a semiconductor element, a thin film transistor is given, for example. Accordingly, the semiconductor device includes liquid crystal display devices, memory devices, and the like.
2. Description of the Related Art
As one kind of a volatile memory, a dynamic random access memory (DRAM) is widely known.
On the other hand, in recent years, metal oxides having semiconductor characteristics (hereinafter, referred to as oxide semiconductors) have attracted attention. Oxide semiconductors can be applied to transistors (see Patent Documents 1 and 2).